North Cascades Geology

Domains of The North Cascades

The early studies of Peter Misch and his students established a general
architecture for the North Cascade’s foundation. The range is
sliced by two major faults separating the rocks into three distinct
blocks. Faults are significant crustal cracks where the rocks on each side have moved relative to each other. The western fault is the Straight Creek Fault; the eastern, the Ross Lake Fault. Because the Ross Lake Fault consists of many fault strands, it is sometimes called the Ross Lake Fault System.

Three major geologic domains make up the North Cascades: the Western, the Metamorphic Core, and the Methow Domains. Heavy lines are major faults.

The Western Domain mostly consists of sedimentary
and volcanic rocks and, although these rocks
are complexly faulted and folded, they still
retain textures and structures, such as sedimentary
bedding, that are typical of their origins on
the surface of the Earth. Many of these rocks
contain fossils.

The central, Metamorphic Core Domain comprises
highly squeezed and recrystallized metamorphic rocks
which were once at great depths in the Earth’s crust. Their story
is complex. If they ever had fossils, the fossils have been destroyed
by the metamorphism,
and how these rocks originally formed can only be
inferred from their compositions and vague relicts of their original
structures and textures.

The Methow Domain is essentially unmetamorphosed
sedimentary and volcanic rocks. They are less complex
than rocks in the other domains and have fossils,
in some places abundantly.

Material in this site has been adapted from a book,
Geology of the North Cascades: A Mountain Mosaic by R. Tabor
and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published
by The Mountaineers, Seattle.